Why does the Remesh Modifier change object outer dimensions?
The problem is resolved by using a closed hemisphere mesh (below left), rather than an open hemisphere mesh (below right).



Using a fully enclosed mesh, the remesh tool retains the outer dimensions of the geometry (without shrinking the input shape) as the Octree Depth is decreased.



Perhaps in future I will close the geometry, apply the Remesh Modifier, and then remove the plane that closes the geometry after the modifier is applied.